This invention relates to electronic circuitry means for eliminating from a seismic signal channel interfering signals of the type that have come to be known as hiline signals. As is well understood in the art of prospecting for underground oil deposits, a number of geophones, or similar devices, are distributed about an area in an array to detect seismic impulses. Long, two-wire electrical cables connect the respective geophones to signal processing equipment at a central location such as a truck or van. Desired seismic signals propagate on a two wire cable in what commonly is called a differential mode. That is, the signal represents a potential on one conductor relative to the potential on the second conductor.
The seismic equipment frequently is in the vicinity of electrical equipment and electrical transmission lines that carry current which is at a frequency of 60 Hertz in the U.S., and other frequencies in some other countries. Ideally, if the electrical cables that carry the seismic signals are electrically balanced with respect to each other and to their surroundings, the 60 Hertz stray currents and fields associated with the power lines and equipment will produce equal voltages in the two conductors of the cable. Signals of this type on the cable are called common mode signals.
High gain input amplifiers couple the seismic signals from the cables to signal processing equipment. These amplifiers are designed and constructed to amplify input signals in the differential mode, but provide up to 100 db, for example, rejection to common mode signals. If ideal conditions existed, the high gain input amplifiers could provide adequate isolation so that the undesired hiline common mode signals would not be present to any troublesome extent at the output terminals of the high gain amplifiers. However, because ideal conditions do not always exist in actual practice, the 60 Hertz currents and fields do not produce equal voltages on the two conductors of a cable. Consequently, an undesired 60 Hertz hiline signal is present on the cable in the differential mode. Because the high gain input amplifier is responsive to signals in the differential mode, the undesired 60 Hertz hiline signal in that mode will be present in the seismic signal that is coupled from the high gain amplifier to the signal processing apparatus.
Various means have been employed in attempts to eliminate the undesired differential mode signals from seismic signal channels. Notch filters have been employed, but they can be difficult and expensive to design and build. Additionally, their characteristics may vary in time. Furthermore, they may introduce phase shifts or other adverse effects into the seismic signals channels.
Other techniques are known for generating a signal that is substantially equal in amplitude and opposite in phase to the interfering signal. This generated signal is combined with the interfering signal to buck it out, or null it.
There are other approaches which attempt to precisely electrically balance the two lines of the cable so that the interfering 60 Hertz signal will appear on the cable only in the common mode. This signal then will be substantially eliminated by the high gain input amplifier that has very high common mode rejection characteristics. The present invention is of the latter type.